New Zealand reported 14 new Covid-19 cases every day after its biggest city, Auckland, re-locked the door.
The discovery of four new family members infected earlier this week shocked a country that had not recorded local transmission for more than three months.
Of the new cases, 13 were back related to the family, while one was a foreigner who arrived on quarantine.
A three-day lockout was introduced in Auckland on Wednesday.
Prime Minister Jacinda Ardern said in a press conference: “We can see the severity of the situation we are facing.
“It is being dealt with urgently but calmly and methodically.”
All confirmed cases will be under quarantine, officials said. Some of their contacts may also be quarantined.
New Zealand has received international acclaim for its early response to the pandemic.
It is again rapidly moving to reintroduce Auckland strict travel restrictions and nationwide social outliers.
Before Tuesday, New Zealand had spent 102 days without having recorded Covid-19 infections transmitted domestically, one of the few countries to reach such an important milestone.
Ms. Ardern also said she hopes the outbreak will develop more before it slows down.
How did the virus reappear in New Zealand?
Health care workers are racing to find the source of this latest case, examining the family and work relationships of the newcomers.
National Health Director Ashley Bloomfield told Radio Newstalk ZB: “We are doing our best to do the contact tracking we need to do and trace back to find the source of the infection. What is this ”.
Other countries that lifted the ban on the use of Covid-19 after seeing a decrease in new cases have also seen infections rise again.
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Vietnam recorded 99 days without any transmission in the community before a new wave of infections hit Da Nang city in late July.
Researchers there are also searching for the source of the latest outbreak to help stop the outbreak.
Some health experts in New Zealand, as well as in Vietnam, believe that the virus may have circulated in the community for several weeks before new cases are detected.
Dr. Bloomfield previously raised the possibility that the virus had arrived in New Zealand by air when one of the infected family members worked in a frozen cargo import warehouse.
But on Thursday he said it was “a low possibility” without detailing other potential sources, according to Reuters.
Now in Auckland there are familiar closures as people flock to supermarkets to buy food. Other local media reported that others in the city were waiting for hours to be tested for the virus.
The spike in new cases has increased the likelihood of a prolonged strike in the city, as New Zealand is only weeks away from scheduled elections.
Ms. Ardern will announce her next steps on the close on Friday and decide on the poll by Monday.